Photochemistry of 1,3-Dioxolane Radical Cations in Sulfur Hexafluoride and Freonic Matrices at 77 K

The elementary stages and efficiency of the photochemical reactions of 1,3-dioxolane radical cations in various low-temperature matrices (sulfur hexafluoride, Freon-11, Freon-113) were determined. The matrix effects in the photochemical processes were observed experimentally. Possible reasons for these effects are discussed.     

Photochemical transformations of cyclic acetal radical cations in Freon matrices at 77 K

The efficiency of photochemical reactions of radical cations of cyclic acetals (1,3-dioxolane, 1,3-dioxane) is measured in different Freon matrices at 77 K and the influence of the latter on the reaction path is discovered. The possible nature of the paramagnetic complexes that form in photochemical reactions of cyclic acetal radical cations in Freon-11 is suggested.     

Reactions of the primary radical cations of carboxylic acids as studied by ESR in freonic matrices

Radical products of radiolysis of frozen solutions of propionic and butyric acids were studied in the matrices of Freon-11, Freon-113, and Freon-113a at 77 K. It was shown that the primary radical cations generated by radiation were not trapped in the freonic matrices (in contrast with the corresponding freonic solutions of acetic acid). The radical cations of propionic and butyric acids decay in concurrent processes of rearrangements yielding terminal-type and ylide-type distonic radical cations and intramolecular proton transfer in the dimeric radical cations resulting in acyloxy radicals. The latter species undergo decarboxylation to yield ethyl and propyl radicals for propionic and butyric acids, respectively. According to mass-spectrometric data, the terminal-type distonic radical cations undergo the McLafferty rearrangement.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 2, 2005, pp. 97–104.Original Russian Text Copyright © 2005 by Belevskii, Belopushkin.This revised version was published online in April 2005 with a corrected cover date.     

Experimental and theoretical study on the structure and reactions of 1-methoxypropane radical cations

Structure and mechanism of thermal and photochemical reactions of radical cations of methyl n-propyl ether (MPE) were studied in irradiated freonic matrices CFCl₃, CF₂ClCFCl₂, and CF₃CCl₃ at 77 K. The quantum chemical calculations of the structure of radical cations and products of their transformations were carried out with methods based on the density functional theory (DFT). Experimental and calculation results show that the MPE radical cations are characterized by substantial delocalization of spin density to the propyl group. The action of light on the MPE radical cations in a CF₃CCl₃ matrix at 77 K results in intramolecular rearrangement yielding the distonic radical cation ^.CH₂CH₂CH₂(OH⁺)CH₃. It was found that the primary MPE radical cations underwent irreversible transformation to CH₃CH₂CH₂OCH ₂ ^. radical as a result of an ion-molecule reaction that occurred in a CF₂ClCFCl₂ matrix upon heating the sample to 110–120 K or in a CFCl₃ matrix upon increasing the solute concentration.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 2, 2005, pp. 105–113.Original Russian Text Copyright © 2005 by Belevskii, Feldman, Tyurin.This revised version was published online in April 2005 with a corrected cover date.     

Stabilization and reactions of tetramethylurea radical cations in Freon matrices at 77 K: Intramolecular hydrogen transfer,ion-molecular reactions,and fragmentation by electron spin resonance study

ESR spectra for -irradiated at 77 K solutions /0.02–16%/ of tetramethylurea /TMU/ in CFCl₃ and Freon-113 have been studied. TMU^+. radical cations radiolytically produced in dilute solutions have been shown to undergo intramolecular hydrogen transfer upon photobleaching resulting in CH₂N= type radical. Evidence for intermolecular proton transfer in TMU^+. radical cations after annealing to phase transition temperature /110–120 K/ in Freon-113 was obtained. Primary radical cations of TMU^+. at their ground state take part in ion-molecular reaction via proton transfer. Molecular cations in their excited states may undergo fragmentation producing Me₂N radicals, which were trapped in liquid phase by t-BuNO as a spin trap.     

Photochemistry of Ethylbenzene Radical Cations in Low-Temperature Freonic Matrices

Two different conformers of ethylbenzene radical cations (or a mixture of both conformers) can be stabilized in various freonic matrices. The first conformer retains the geometry of the parent molecule, whereas the second one corresponds to minimum energy. It was shown that the photochemical reactions of the radical cations in various freons at 77 K were not accompanied by a change in their conformational state. The spectral characteristics of ethylbenzene radical cations and the quantum yields of photoinduced charge transfer reactions were determined. The reasons for stabilization of different conformers of the radical cations in different freonic matrices are discussed.     

Spectral Characteristics and Transformations of Intermediates in Irradiated Freon 11, Freon 113, and Freon 113a

Optical absorption bands observable in Freon 11, Freon 113, and Freon 113a irradiated at 77 K were assigned to various intermediates (radical cations, radical ion pairs, and complexes of radicals with ions). The transformations of these species in thermal and photochemical reactions occurring at 77 K were studied. On the basis of experimental results, it was suggested that the radical anions of Freon 11 and Freon 113 are unstable at 77 K and the spatial distribution of the intermediates produced is inhomogeneous.     

Ion-molecule reactions of primary radical cations in the liquid-phase radiolysis of <Emphasis Type="Italic">n</Emphasis>-alkanes: An EPR study

Radiation-chemical yields the liquid-phase radiolysis of C₅–C₁₂ n-alkanes were measured using the spin trap technique. The yields of n-alkyl radicals depended only slightly on the chain length in C₅–C₉ alkanes and amounted up to 30% of the total yield of trapped radicals; they were inhibited by the addition of charge scavengers. An analysis of the experimental results together with data on radicals in irradiated crystalline alkanes and radical cations in freon matrices showed that n-alkyl radicals results from the ion-molecule reactions of primary radical cations, whereas the protonated ions RH₂⁺ as products of these reactions are a source of sec-alkyl radicals. At least 60% of primary radical cations are consumed via these reaction pathways. A part of sec-alkyl radicals is due to gauche-conformers. The relative amount of primary alkyl radicals formed in the degradation of excited states and the subsequent charge neutralization processes should be insignificant.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 1, 2005, pp. 5–14.Original Russian Text Copyright © 2005 by Belevskii, Belopushkin.     

Ion-molecule reactions and thermal isomerization of tricyclo[4.3.0.0]nona-4,8-diene radical cations to tricyclo[4.2.1.0]nona-2,7-diene radical cations in a γ-irradiated frozen Freon matrix

A four-step mechanism of isomerization of tricyclo[4.3.0.0^3,7]nona-4,8-diene radical cations to tricyclo[4.2.1.0^4,9]nona-2,7-diene radical cations in γ-irradiated frozen Freon-113 (CFCl₂CF₂Cl) matrix was suggested on the basis of ESR data. The rearrangement was found to occur via distonic form of the radical cations with spin and charge separation. Furthermore, it was shown that the primary radical cations abstracts hydrogen atom from methylene group of the parent molecule, whereas distonic radical cations reacts via attachment to the C=C bond at 110–119 K.     

ON THE ORIGIN OF LASER-SPECIFIC LUMINESCENCE UPON PULSED-LASER EXCITATION OF BENZOPHENONES IN FREONS†

–Excimer laser excitation (308 nm) of benzophenone in Freon-113, as well as in Freon-11, results in an intense emission with λ_max at approx. 530 nm in addition to triplet phosphorescence. This emission is shown to result from an excited free radical bearing the diphenylmethylene moiety which is generated in a two-photon process and which luminesces upon sensitization by the benzophenone triplet. On the basis of its spectroscopic properties and the chemical nature of the system, it is suggested that the fluorescer in this system may be the hypochlorite radical, Ph₂COCl.     

Photochemistry of trimethylene oxide and trimethylene sulfide radical cations in freonic matrices at 77 K

It was shown that trimethylene oxide (oxetane) radical cations were converted at 77 K into either distonic radical cations ·CH₂CH₂CH=OH⁺ or 2-oxetanyl radicals, depending on the freonic matrix used, by the action of light at λ = 546 nm and trimethylene sulfide radical cations transformed into distonic radical cations CH₂CHSH⁺CH ₂ ^· under 436-nm irradiation. The quantum yields of the photochemical reactions were determined. Quantum-chemical calculations on the structure and HFC constants of the radical cations and possible paramagnetic products of their transformation were performed. The reasons behind the observed difference in reactivity between the radical cations under the action of light are discussed.     

Mobile protons versus mobile radicals: Gas-phase unimolecular chemistry of radical cations of cysteine-containing peptides

A combination of electrospray ionization (ESI), multistage, and high-resolution mass spectrometry experiments are used to examine the gas-phase fragmentation reactions of radical cations of cysteine containing di- and tripeptides. Two different chemical methods were used to form initial populations of radical cations in which the radical sites were located at different positions: (1) sulfur-centered cysteinyl radicals via bond homolysis of protonated S-nitrosocysteine containing peptides; and (2) α-carbon backbone-centered radicals via Siu’s sequence of reactions (J. Am. Chem. Soc. 2008, 130, 7862). Comparison of the fragmentation reactions of these regiospecifically generated radicals suggests that hydrogen atom transfer (HAT) between the α C-H of adjacent residues and the cysteinyl radical can occur. In addition, using accurate mass measurements, deuterium labeling, and comparison with an authentic sample, a novel loss of part of the N-terminal cysteine residue was shown to give rise to the protonated, truncated N-formyl peptide (an even-electron x_n ion). DFT calculations were performed on the radical cation [GCG]^.+ to examine: the relative stabilities of isomers with different radical and protonation sites; the barriers associated with radical migration between four possible radical sites, [G.CG]⁺, [GC.G]⁺, [GCG.]⁺, and [GC(S.)G]⁺; and for dissociation from these sites to yield b₂-type ions.     

Generation and reactions of 3-alkylidene-1-pyrazoline radical cations by photoinduced electron transfer

The behavior of the 3-alkylidene-1-pyrazoline radical cations generated by photoinduced electron transfer reactions was examined. The nitrogen-retained radical cations have been detected using laser flash photolysis. The photochemical products indicate that E/Z isomerization, intramolecular cyclization, and solvent addition (acetonitrile) occurred.     

Structure and Reactivity of the Radical Cations of Methyl tert-Butyl Ether in Condensed Phase: An ESR and Quantum Chemical Study

The structure of the methyl tert-butyl ether (MTBE) radical cation and mechanism of its thermal and photochemical reactions in irradiated freons (CFCl₃, CF₂ClCFCl₂, and CF₃CCl₃) were studied. Radical products of MTBE radiolysis in the liquid phase were investigated by the spin trapping technique. The quantum-chemical calculations of the structure of MTBE radical cations and products of their transformations were carried out by density functional theory (DFT) and ab initioMP2 methods. The primary MTBE radical cations are stabilized in dilute solutions in CFCl₃and CF₃CCl₃. The ion–molecule reaction (proton transfer from the radical cation) was found to occur in concentrated solutions in CFCl₃immediately during irradiation. The action of light ( = 436 to 546 nm) at 77 K on the MTBE radical cation in CFCl₃and CF₃CCl₃matrices results in intramolecular migration of the methyl group to yield the distonic radical cation (CH₃)₂ ^.CO⁺(CH₃)₂. The primary MTBE radical cations undergo an irreversible transformation with methane elimination resulting in formation of the 2-methoxypropene radical cation ^.CH₂=⁺(₃)₃in CFCl₃and CF₃CCl₃matrices in the temperature range 110–130 K. In the case of CF₂ClCFCl₂matrix, such a reaction occurs during irradiation at 77 K. Using the spin trapping technique, it was shown that the liquid-phase radiolysis of the neat ether resulted in the formation of fragmentation products (^.CH₃,CH₃^., and t-BuO^. radicals) from the primary radical cations, as well as the products of their rearrangements and ion–molecule reactions.     

Photochemical Hydrogen Evolution at Metal Centers Probed with Hydrated Aluminium Cations,Al+(H2O)n,n=1–10

Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molecules with loss of atomic hydrogen, molecular hydrogen and water molecules. Quantum chemical calculations for n=2 reveal that solvation shifts the intense 3s–3p excitations of Al⁺ into the investigated photon energy range below 5.5 eV. During the photochemical relaxation, internal conversion from S₁ to T₂ takes place, and photochemical hydrogen formation starts on the T₂ surface, which passes through a conical intersection, changing to T₁. On this triplet surface, the electron that was excited to the Al 3p orbital is transferred to a coordinated water molecule, which dissociates into a hydroxide ion and a hydrogen atom. If the system remains in the triplet state, this hydrogen radical is lost directly. If the system returns to singlet multiplicity, the reaction may be reversed, with recombination with the hydroxide moiety and electron transfer back to aluminium, resulting in water evaporation. Alternatively, the hydrogen radical can attack the intact water molecule, forming molecular hydrogen and aluminium dihydroxide. Photodissociation is observed for up to n=8. Clusters with n=9 or 10 occur exclusively as HAlOH⁺(H₂O)_n-1 and are transparent in the investigated energy range. For n=4–8, a mixture of Al⁺(H₂O)_n and HAlOH⁺(H₂O)_n-1 is present in the experiment.     

Photochemical transformations of 1,3,5-trioxane radical cations in freonic matrices at 77 K

It was found that the principal photochemical reaction of 1,3,5-trioxane radical cations in freonic matrices at 77 K is their cycle-opening dissociation yielding the distonic radical cation in which the unpaired electron is preferentially localized on the oxygen atom. The dissociation of the trioxane radical cations at 77 K is characterized by high quantum yields, which vary from 0.24 to 0.36 in different matrices. The distonic radical cations produced during photolysis are unstable at 77 K and undergo further transformations, which occur at different rates in freonic matrices. The structure of the intermediates produced and a possible mechanism of the processes are discussed with the use of quantum-chemical calculation data.     

Fluorous solvent‐soluble imaging materials containing anthracene moieties

Highly fluorinated photoresist polymers that can undergo photodimerization reactions were designed using an anthracene‐based monomer. Through the random radical copolymerizations of 6‐(anthracen‐9‐yl)hexyl methacrylate ( AHMA ) and semiperfluorodecyl methacrylate ( FDMA ) with four different compositions, polymers with M_n = 20,000–27,000 (M_w/M_n = 2.0–2.9) were prepared in benzotrifluoride. The polymers, in particular fluorous solvent‐soluble imaging material‐2 ( FSIM‐2 ), showed sufficient solubility in fluorous solvents, including hydrofluoroethers, but were rendered insoluble by UV exposure (365 nm). This photochemical solubility change was evaluated quantitatively by a quartz crystal microbalance technique, along with tracing the chemical reaction by UV–vis spectroscopy. Finally, FSIM‐2 and fluorous solvents were applied to the photolithographic patterning of organic light‐emitting diode pixels. In the patterning protocol involving the lift‐off of resist films in fluorous solvents, FSIM‐2 was recognized as a promising photoreactive material when compared with a reference polymer P(FDMA‐MAMA) , which necessitates acidolysis reactions for lithographic imaging. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1252–1259     

N‐methylpicolinium derivatives as the coinitiators in photoinitiating systems for vinyl monomers polymerization

Five N‐methylpicolinium derivatives were investigated to test their abilities to function as second coinitiators in free radical photopolymerization initiated by N,N′‐diethylcarbocyanine—n‐butyltriphenylborate photoredox pair ( P19B2 ). As it is shown by the kinetic data, an addition of picolinium derivatives into P19B2 photoinitiating system visibly increases the efficiency of photoinitiation. The results suggest that the rates of photoinitiation depend on the rate of the picolyl radicals formation. The redox potentials of tested N‐methylpicolinium derivatives were measured and the calculation of free energy change for the possible electron transfer reactions between all components of the system (both stable and transient individuals) was performed. The results suggest that cyanine dyes are able to start a specific chain of an electron transfer reactions involving different coinitiators (borate salt and N‐alkylpicolinium derivatives), giving as a result one photon—two‐radicals photochemical response. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 576–588, 2009     

Rate constants for the reactions of OH radicals and Cl atoms with Di‐n‐Propyl ether and Di‐n‐Butyl ether and their deuterated analogs

Using relative rate methods, rate constants for the gas‐phase reactions of OH radicals and Cl atoms with di‐n‐propyl ether, di‐n‐propyl ether‐d₁₄, di‐n‐butyl ether and di‐n‐butyl ether‐d₁₈ have been measured at 296 ± 2 K and atmospheric pressure of air. The rate constants obtained (in cm³ molecule⁻¹ s⁻¹ units) were: OH radical reactions, di‐n‐propyl ether, (2.18 ± 0.17) × 10⁻¹¹; di‐n‐propyl ether‐d₁₄, (1.13 ± 0.06) × 10⁻¹¹; di‐n‐butyl ether, (3.30 ± 0.25) × 10⁻¹¹; and di‐n‐butyl ether‐d₁₈, (1.49 ± 0.12) × 10⁻¹¹; Cl atom reactions, di‐n‐propyl ether, (3.83 ± 0.05) × 10⁻¹⁰; di‐n‐propyl ether‐d₁₄, (2.84 ± 0.31) × 10⁻¹⁰; di‐n‐butyl ether, (5.15 ± 0.05) × 10⁻¹⁰; and di‐n‐butyl ether‐d₁₈, (4.03 ± 0.06) × 10⁻¹⁰. The rate constants for the di‐n‐propyl ether and di‐n‐butyl ether reactions are in agreement with literature data, and the deuterium isotope effects are consistent with H‐atom abstraction being the rate‐determining steps for both the OH radical and Cl atom reactions. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 425–431, 1999     

Spectroscopic characterization of alkane radical cations—I. Electronic absorption spectra of 3-methylalkane radical cations

Radical cations of various 3-methylalkanes (C₆-C₁₄) have been produced and stabilized by γ-irradiation of the corresponding neutral compounds in saturated chloroflourocarbon (1,1-diflourotetra-chloroethane and 1,1,2-trichlorotriflouroethane) and perflourocarbon (perflourohexane and perfluoro-methylcyclohexane) matrices at 77 K. The perfluorocarbon matrices appeared more suitable for studies of the lighter radical cations, whereas the chlorofluorocarbon matrices were more suited for studies of the heavier radical cations; intermediary cations could be studied in both types of matrices. After irradiation, electronic absorptions associated with both the matrix and the alkane additive were observed. Pure spectra of the 3-methylalkane radical cations were obtained by difference spectrometry, after selective elimination of these cations by illumination. The electronic absorption spectra of the 3-methylalkane radical cations consist in all cases of a single broad absorption band. The spectral position of this band shifts to longer wavelengths with increasing chain length; the maximum of the absorption band was found to be situated at 490 nm for 3-methylpentane radical cations and at 940 nm for 3-methyltridecane radical cations. The results are most interesting because they give direct information on the electronic absorption of 3-methylpentane radical cations. It was found that the molar extinction coefficients of these cations are not very much smaller than those of other 3-methylalkane radical cations and thus must be of the order of 10³dm³·mol^-1·cm^-1. From this it is deduced that the majority of positive ions trapped in irradiated pure 3-methylpentane glasses at 77 K are not parent cations.